Arm64 CRC32 compiling warning suppression

[guyuqi]

When 'HAS_ARMV8_CRC' is enabled, 'rocksdb::crc32c::isPCLMULQDQ() / isSSE42()'
will not be used but have been actually defined.
So it's needed to suppress the warning here.

[miasantreble]

is this change related to #5304? do we really need -Wno-unused-function flag in both places?

[Yikun]

@miasantreble I think we still need this patch in the build of linux arm64. Otherwise, we will get back the error like when do make -j8 rocksdbjava:

util/crc32c.cc:404:13: error: ‘bool rocksdb::crc32c::isSSE42()’ defined but not used [-Werror=unused-function]
 static bool isSSE42() {
             ^
util/crc32c.cc:420:13: error: ‘bool rocksdb::crc32c::isPCLMULQDQ()’ defined but not used [-Werror=unused-function]
 static bool isPCLMULQDQ() {
             ^
cc1plus: all warnings being treated as errors
Makefile:1904: recipe for target 'jl/util/crc32c.o' failed
make: *** [jl/util/crc32c.o] Error 1

And the other potential choice is add the ifndef on these two method.

~/rcoksdb$ git diff util/crc32c.cc
diff --git a/util/crc32c.cc b/util/crc32c.cc
index e8d4116..16237ac 100644
--- a/util/crc32c.cc
+++ b/util/crc32c.cc
@@ -398,6 +398,7 @@ uint32_t ExtendImpl(uint32_t crc, const char* buf, size_t size) {
   return static_cast<uint32_t>(l ^ 0xffffffffu);
 }
 
+#ifndef HAVE_ARM64_CRC
 // Detect if SS42 or not.
 #ifndef HAVE_POWER8
 
@@ -436,6 +437,7 @@ static bool isPCLMULQDQ() {
 }
 
 #endif  // HAVE_POWER8
+#endif  // HAVE_ARM64_CRC

Both these two ways can work in my env, : ).

[guyuqi]

@miasantreble
Sorry for late response.
I think it makes sense to set the flag to 2 places for independent build system(make / cmake).

[siying]

@Yikun can we do the second way?

[Yikun]

@siying Sure, It's OK to me.

I submitted a PR #5565 , and if @guyuqi have no objection, it would be good merge it, or just squash it into this patch.